1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method. In particular, the present invention relates to an image forming apparatus based on a system in which a plurality of developing devices is disposed around a photosensitive drum, and a toner image formed on the photosensitive drum is collectively transferred onto a transferring material after each color is superposed on an intermediate transferring member.
2. Related Background Art
Heretofore, such color image forming apparatuses based on an electrophotographic system have been known that comprise a second image bearing member such as an intermediate transferring member in addition to a first image bearing member such as a photosensitive drum. In these apparatuses, a so-called primary transfer is performed for transferring a toner image formed on the first image bearing member onto the second image bearing member, and this primary transfer step is repeated more than once so as to superpose the toner images of a plurality of colors on the second image bearing member, before collectively secondary-transferring these toner images of a plurality of colors onto a conveyed transferring material such as a sheet, and then the toner images are fixed onto the transferring material by a fixing device through melting and pressure fixing.
FIG. 4 shows an example of the image forming apparatus constituted as above which uses an intermediate transferring belt (intermediate transferring member) as the second image bearing member.
A photosensitive drum 3 that rotates in a direction of an arrow A is evenly charged by a charger 5, and an electrostatic latent image is formed by a laser light 6.
Three developing devices 1-1, 1-2 and 1-3 which store toners of colors including three colors of Y, M and C respectively, and a developing device 1-4 which stores a toner of Bk are disposed around the photosensitive drum 3.
One of these developing devices 1-1, 1-2 and 1-3 is selected to come close to the photosensitive drum 3 by changing means 4, while the developing device 1-4 is always close thereto, and one of these two developing devices close thereto is used for developing the electrostatic latent image on the photosensitive drum 3, thereby forming a toner image on the photosensitive drum 3.
As to the arrangement constitution of the developing devices described above, the diameter of the photosensitive drum becomes large if all the four developing devices of Y, M, C and Bk are constituted to be close to the circumference of the photosensitive drum, which leads to a size increase of the apparatus and increased costs, so that the developing device containing the frequently used Bk toner is always close to the circumference of the photosensitive drum, while, as to the three developing devices of Y, M and C, one of these three developing devices is automatically selected to come close to the photosensitive drum, thereby achieving a smaller diameter of the photosensitive drum in accordance with this constitution.
Furthermore, a one-component developing method using a magnetic one-component toner is applied to the Bk (black) which is mostly used for text information and frequently used with demands for lower costs, and a two-component developing method comprising a non-magnetic toner and a magnetic carrier is applied to the Y (yellow), M (magenta) and C (cyan) in response to demands for improved image quality, thus combining the two developing methods.
Next, a bias is applied to a primary transferring roller 8 so that a charge having a polarity reverse to that of the toner is given onto a rear surface of an intermediate transferring belt 7, and the toner image developed on the photosensitive drum 3 is primary-transferred onto the intermediate transferring belt 7 via a primary transferring portion (primary transferring nip) 9. A primary transferring residual toner remaining on the surface of the photosensitive drum 3 that has finished the primary transfer is removed and collected by a cleaning member 13, and further residual charge is removed by an exposure 14, and then an image forming process of a next color is started.
This primary transferring step is repeated for the toner images of the four colors, thus forming a full color toner image with the four colors superposed on the intermediate transferring belt 7. Then, a charge having a polarity reverse to that of the toner is given from a secondary transferring outer roller 10b to which a bias is applied onto a rear surface of a transferring material 12 which is held and conveyed by a secondary transferring portion 11 formed among a secondary transferring inner roller 10a, the intermediate transferring belt 7 and the secondary transferring outer roller 10b, so as to secondary-transfer the above full color toner image collectively onto the transferring material 12. The full color toner image is fixed by an unillustrated fixing device to obtain an image on the transferring material 12. A secondary transferring residual toner remaining on the intermediate transferring belt 7 that has finished the above-described secondary transfer is removed by an unillustrated cleaning member.
Rollers having a resistance of equal to or less than 1010 Ω·cm are generally used as the above primary transferring roller and secondary transferring roller.
For the above intermediate transferring belt 7, a resin belt with no end having a thickness of about 50 to 300 μm in which resistance is adjusted to have a volume resistivity of about 1011 to 1016 Ω·cm can be used, as an example. For instance, a resin film such as PVdF (polyvinylidene fluoride), nylon, PET (polyethylene terephthalate) or polycarbonate can be used for the material of the resin belt. Regarding the resistance adjustment, it is possible to adjust the volume resistivity to about 108 to 1012 Ω·cm by using carbon, ZnO, SnO2, TiO2 or other conductive filling materials for the above resin belt. By keeping the resistance to a low to intermediate level in this way, poor image quality due to the accumulation of charge in the intermediate transferring belt 7 can be prevented, and a charge removing system can be dispensed with.
Furthermore, as another example, a rubber material (chloroprene rubber, EPDM, NBR, urethane rubber, etc.) having a hardness lower than that of the resin and a thickness of about 0.5 to 2 mm can be used for the material of the intermediate transferring belt 7 after adjusting it to have a volume resistivity of about 1011 to 1016 Ω·cm.
The image forming apparatus above has a one-image mode for forming a toner image of one sheet of transferring material (α) on the intermediate transferring belt 7 and a two-image mode for forming toner images of two sheets (α) and (β), and herein, a case of the two-image mode will be described as an example of changing the order of forming the toner images on the photosensitive drum.
First, toner images (αY) and (βY) of a single color Y are developed in this order on the photosensitive member 3 by the developing device 1-1 selectively close to the photosensitive member 3, and then primary-transferred onto the intermediate transferring belt 7 as shown in FIG. 5A, and while the developing device to be selectively close to the photosensitive member 3 is being changed from the developing device 1-1 to the developing device 1-2, only a toner image (αk) of a single color Bk is developed on the photosensitive member 3 by the developing device 1-4 which is always close to the photosensitive member 3, and then the toner image (αk) is superposed on the toner image (αY) of the single color Y on the intermediate transferring belt 7 for the primary transfer, thereby forming a toner image (αYk) on the intermediate transferring belt 7 as shown in FIG. 5B.
Toner images (βM) and (αM) of a single color M are developed in this order on the photosensitive member 3 by the developing device 1-2 which has been changed and come close to the photosensitive drum 3, and then are sequentially superposed on the toner image (βY) and the toner image (αYk) on the intermediate transferring belt 7 for the primary transfer, so as to form toner images (βYM) and (αYkM) on the intermediate transferring member as shown in FIGS. 5C and 5D, and then, while the developing device to be selectively close to the photosensitive member 3 is being changed from the developing device 1-2 to the developing device 1-3, only a toner image (βk) of the single color Bk is developed on the photosensitive member 3 by the developing device 1-4 which is always close to the photosensitive member 3, and then the toner image (βk) is superposed on the toner image (βYM) on the intermediate transferring belt 7 for the primary transfer so as to form a toner image (βYMk) on the intermediate transferring member as shown in FIG. 5E, and toner images (αC) and (βC) of a single color C are developed in this order on the photosensitive member 3 by the developing device 1-3 which has been changed and come close to the photosensitive drum, and then are sequentially superposed on the toner images (αYkM) and (βYMk) on the intermediate transferring belt 7 for the primary transfer, so as to form toner images (αYkMC) and (βYMkC) on the intermediate transferring member as shown in FIGS. 5F and 5G, and the toner images (αYkMC) and (βYMkC) on the intermediate transferring belt 7 formed by the above repeated primary transferring step are secondary-transferred collectively onto the transferring material 12. The above constitution effectively uses the time when the developing devices of Y, M and C are changed to produce the image of Bk, thereby reducing the time needed for image formation.
Incidentally, it is generally known that a charge amount (triboelectricity) of the Bk toner is smaller than that of the Y, M and C toners. This is attributed to magnetic substances and carbon contained in the Bk toner. It is also known that an optimum transferring bias increases in proportion to the charge amount of the toner. Further, it is also known that the triboelectricity is less in a magnetic Bk toner than in a non-magnetic Bk toner even in the case of the Bks of the same color.
FIG. 6 and FIG. 7 are graphs schematically representing optimum secondary transferring biases in terms of the kind and state of toners. FIG. 6 shows a case where the toners on the photosensitive member are not charged, while FIG. 7 shows a case where the toners on the photosensitive member are charged.
In such states, when the non-magnetic YMC toner and the magnetic Bk toner on the intermediate transferring belt 7 are to be transferred collectively by the secondary transferring nip portion 11, if the optimum secondary transferring bias of the YMC toner is applied to the secondary transferring bias, a poor secondary transfer is caused because the bias is too high for the Bk toner, as shown in FIG. 6, on the other hand, if the optimum secondary transferring bias of the Bk toner is applied to the secondary transferring bias, a poor secondary transfer is caused because the bias is too low for the YMC toner.
Therefore, heretofore, the charge amount of the magnetic Bk toner image on the photosensitive member 3 is increased by a post charger 2 illustrated in FIG. 4 so that the optimum secondary transfer can be applied to both the magnetic toner and the nonmagnetic toner as shown in FIG. 7.
In addition, Japanese Patent Application Laid-Open No. 11-231597 discloses a configuration in which a pair of images on a front surface and a rear surface is formed in two image forming portions, and when both the surfaces are collectively transferred simultaneously, the charge amounts of the pair of images on the front and rear surfaces are adjusted to almost correspond by the charger provided in the drum.
However, the above conventional example has the following problems.
While the toner transferred onto the intermediate transferring belt 7 from the photosensitive drum 3 continues to be retained on the intermediate transferring belt 7 even during the first transferring step repeated after that, the charges are given and received between the toner, and the photosensitive drum 3 or the intermediate transferring belt 7, in the primary transferring nip portion 9, and even of the toners of the same color, the toner that has a greater number of times of passing through the primary transferring nip portion has a larger charge amount (triboelectricity) shortly before the secondary transfer, as shown in FIGS. 8A and 8B.
It is known that the optimum secondary transferring bias also increases in proportion to the charge amount of the toner, as described above, and this results in different optimum secondary transferring biases when the secondary transfer is performed with two times of passing through the primary transferring nip portion and when the secondary transfer is performed with three times of passing. In the conventional example, the number of times of passing through the primary transferring nip portion 9 in association with the non-magnetic toner images is two for (αM) and three for (βM), and in association with the magnetic toner images, three for (αk) and two for (βk).
However, as shown in FIG. 9, since the non-magnetic toners originally have a smaller difference of triboelectricity between themselves, it is possible to obtain a common optimum secondary transferring bias even if the number of times of passing is different.
However, the magnetic toners (αk) and (βk) have a larger difference of charge amount changes in relation to the number of times of passing than the non-magnetic toners as shown in FIG. 8B, and when the same amount of charge is applied to the above magnetic Bk toner images (αk) and (βk) by the above post charger 2 illustrated in FIG. 4, either the magnetic toner (βk) that passes through the primary transferring portion 9 twice or the magnetic toner (αk) that passes three times might cause a poor secondary transfer. More specifically, either the full color toner image (α) or (β) constituting the toner image of two sheets on the intermediate transferring member might cause a poor secondary transfer during the secondary transfer.